reserve V,X,Y for RealLinearSpace;
reserve u,u1,u2,v,v1,v2 for VECTOR of V;
reserve a for Real;
reserve V1,V2,V3 for Subset of V;
reserve x for object;
reserve W,W1,W2 for Subspace of V;
reserve w,w1,w2 for VECTOR of W;

theorem Th16:
  w1 = v & w2 = u implies w1 - w2 = v - u
proof
  assume that
A1: w1 = v and
A2: w2 = u;
  - w2 = - u by A2,Th15;
  hence thesis by A1,Th13;
end;
